In many telecommunications applications, repeaters and other electronic devices are housed in remote units scattered throughout a geographical region in the vicinity of a central office. In one example, a remote unit communicates with the central office and also receives power from the central office through the same cable or other communication medium. This cable is also referred to as a “span cable.” An example of a span cable includes, a span cable comprising a set of twisted-pair conductors over which telecommunications data is transferred between the central office and the remote units, and over which DC power is supplied by the central office to the remote unit. The remote unit typically utilizes the power received from the central office over the span cable to power one or more electronic devices within the remote unit. The power delivered via a span cable is often susceptible to disturbances (such as faults, voltage spikes and surges) caused by environmental factors such as lighting and nearby electrostatic discharges. Left unmitigated, such power disturbances can interrupt telecommunications operations and permanently damage equipment.
A variety of lightning arrestors, surge protectors, and the like are available to mitigate voltage and other power disturbances. When installed in a span-powered remote unit, at least one such protective device protects the electronic devices within the remote unit by shunting the surge away from the electronic devices. One approach is to install one or more “crowbarring” solid-state protective devices between the conductors of a span cable or between conductors of the span cable and ground. During normal operation with nominal voltage across the protective device, the protective device behaves as a high impedance (that is, an “open”) element through which current does not flow. However, when the voltage across the protective device exceeds a certain threshold (for example, due to a disturbance related to the span cable), the device becomes a short circuit, protecting the electronic devices by shunting current away from the electronic devices. Often, such a protective device is designed to remain shorted in this way as long as a holding current continues to flow through the protective device. Examples of such crowbarring solid-state surge protection devices include, but are not limited to SIDACTOR devices manufactured by Littlefuse, Inc.
However, current provided from a power supply at the central office typically will continue to feed current through a shorted protective device, preventing the protective device from resetting itself, even after the power disturbance has passed. In such a situation, to reset the protective device, the power supply at the central office is typically powered down and then turned back on (that is, the power supply is “rebooted”). While the protective device is shorted, the limited power received from the span cable at the remote unit is not sufficient to power the remote unit. Also, while the power supply is rebooting, power is not supplied on the span cable for powering the remote unit. At the central office, the entire process of detecting an “on” (that is, shorted) protective device, rebooting the power supply, and returning the system to a data carrying mode, can take several seconds. Remote units typically only maintain sufficient capacitive energy storage to ride through a loss of span power in the range of 15 to 20 milliseconds, and the installation of additional capacitive storage is prohibitively expensive. When such stored energy is exhausted, any communication link between the central office and the remote unit will typically go down. As a result, even span-power disturbances of only a few milliseconds in duration can result in the loss of span power between a central office and a remote unit for several seconds, and the loss of any communication link between them as well.